The invention relates generally to a method for laser welding and more particularly for the laser welding of highly reflective fins and tubes.
Laser welding is a widely applied method for both high-volume and small-batch welding of many different metal products. Due to the small beam spotsize, laser welding is also an ideal process for thin-section metal welding and sheet metal applications where the extent of the heat-affected zone must be minimal. These attributes are both also especially important for the joining of fin and tube solar collectors. Unfortunately, the general use of copper or aluminum in solar collectors presents difficulties in utilizing laser welding because the laser beam is easily reflected from the weld pool surface. The reflectivity of copper to 10.6 .mu.m light is on the order of 99% and production applications involving laser welding of copper and copper alloys are quite rare. When welding materials with high reflectivity, such as aluminum and copper alloys, it is difficult to obtain the depth of penetration achieved with materials such as steel and nickel alloys. While successful laser welds can be made on reflective metals, the process is not robust and can produce defective welds and damage the laser or even the laser operator. Problems such as variable weld penetration, difficulty in bridging gaps, non-uniform weld-bead appearance, spatter, burn-though, and general poor weld quality are often traced to inconsistent laser beam absorption at the workpiece.
The most consistent laser welds, especially on reflective materials, are obtained when a deep, laser-created, metal-vapor-supported cavity is formed in the metal workpiece. The cavity traps the incident laser beam due to multiple internal reflections within. This mechanism, known as the Mendenhall wedge effect, relies on Fresnel absorption where a specific fraction of the beam energy is deposited at each reflecting surface. For shallow depth welds where a deep cavity cannot be formed, specially prepared weld joints must be fabricated to promote multiple reflections to assure consistent absorption.
Achieving desired joint fusion such as where a thin fin joins a relatively thicker wall tube in a solar collector, is difficult and often relies on special joint design and use of multiple weld passes. Rudd (U.S. Pat. No. 4,362,921) describes a method of welding a planar solar panel element to a tube using highfrequency, electric currents wherein the tubing is formed with a pair of abutting lips to facilitate the welding process. Orr (U.S. Pat. No. 3,999,029) also describes a process of welding a fin to a tube using high-frequency electrical resistance heating, facilitating the welding by first deforming the tube.
High energy density welding methods such as electron beam welding and laser welding have been used with consistent energy absorption by achieving high depth-to-width aspect ratio welds. These methods generally utilize keyhole melting and do not require joint preparation and back fill. However, when these methods are used for higher aspect ratio welds, there are increased chances for cold shut voids, root porosity, other root defects and missed joints. Milewski et al. (U.S. Pat. No., 5,760,365) describe a method to overcome problems associated with high depth-to-width aspect ratio welds wherein the weld joint is considered as an optical element and an optical ray tracing technique is used to model a laser beam and join geometry of a weld, calibrating a model through an iterative technique to optimize selection of laser welding parameters to achieve the desired weld. The method provides a complex modeling technique to optimize welding parameters.